This invention relates to a prepolymer and a curative which together form a polyurethane structural adhesive for bonding fiberglass reinforced polyester ("FRP") and metal parts without using a primer pretreatment. These parts are used in the manufacture of transportation vehicles such as cars, trucks and boats.
Polyurethane structural adhesives have been in use for many years and have certain characteristics which make them difficult to use as widely as they might be used. For example, they are sensitive to the degree of moisture in the FRP substrates and may form weaker bonds when the substrates are damp. After the adhesive is applied to the substrate there is often too brief a time period within which the adhesive is still fluid and the second substrate can be adhered. This time period is known as the "open time" and it would be commercially useful to have adhesives whose "open times" could be varied according to the needs of the vehicle manufacturers' assembly lines.
Often the adhesive must be applied to a substrate in a vertical position on which, if the adhesive is too fluid, it will drip or sag before the second substrate can be placed over the adhesive. Thus, an adhesive which does not sag is preferred. This property is known as "non-sag" and is measured by noting the distance the adhesive has flowed at the end of a period of time such as three minutes. The test method used to measure sag is ASTM D2202-84.
Commercial polyurethane adhesives presently used in industry show a very slow rate of early strength development ("green strength"). It would be desirable if an adhesive having faster green strength could be found.
During the manufacturing process certain assembled part laminates are prepared towards the end of the Friday shift and left idle until next Monday. The long delay before heat curing or baking of the assembled parts on Monday morning can result in sacrifice of the performance of certain adhesives. A new adhesive which can tolerate a delayed postbake would be welcomed in the assembly line. Even better would be an adhesive curable at ambient or room temperature and not requiring any heating to form strong bonds. Usually polyurethane adhesives require baking at 121.degree. C. or above for about 30 to 60 minutes in order to be strong enough that, when tested, the substrate itself breaks ("delamination"). When a structural adhesive is said to exhibit 100% substrate delamination, inspection of the broken interface between adhesive and substrate displays only the torn fibers and disrupted surface of the FRP substrate itself throughout the entire area of the interface.
Whether or not adhesives require baking to form strong bonds, the bonds formed must withstand high temperatures during subsequent steps in the manufacturing process. For that reason the laminates made with the adhesive of this invention are tested at 82.degree. C. to learn whether they are so tough that the mode of failure is substrate delamination.
Adhesives have been used for many years in industry to adhere plastic materials. One of the drawbacks has been the problem of shrinkage of the plastic surface at the site of application of the adhesive resulting in deformation of the plastic surface and formation of a shallow dimple or ripple in the otherwise smooth outer surface of the plastic. Since we all expect our automobiles, boats and planes to have sleek smooth exterior surfaces ("Class A") such defects are extremely undesirable. This phenomenon of dimpling or rippling at the site of adhesive application is known in the automobile industry as "bond line read through" and has escaped solution. It would be a great improvement if such dimpling, rippling or surface deformation could be eliminated.
As the production of cars and trucks becomes more sophisticated and more automated, new adhesives must be developed which will
adhere to moist substrates, PA0 display variable "open time", PA0 have "non-sag" properties, PA0 have fast "green strength", PA0 tolerate "delayed postbake", PA0 cure at ambient temperature and delaminate the substrate in failure, and form bonds withstanding 82.degree. C. exposure, all without deforming the smooth outer surface of the FRP substrate. PA0 polymerized diphenylmethane diisocyanate having 80 to 99 parts PA0 monomers, 1 to 20 parts uretonimine oligomer and 0 to 15 parts higher molecular weight oligomers, PA0 2,6-toluene diisocyanate or polymerized 2,6-toluene diisocyanate, PA0 3,3'-dimethyl-4,4'-diisocyanatobiphenyl or polymerized 3,3'dimethyl-4,4'-diisocyanatobiphenyl, or PA0 3,3'dimethyl-4,4'-diisocyanato diphenyl methane or polymerized PA0 3,3'dimethyl-4,4'-diisocyanato diphenyl methane PA0 toluene diamine PA0 1-methyl-3,5-diethyl-2,4-diaminobenzene PA0 1-methyl-3,5-diethyl-2,6 diaminobenzene (also known as DETDA or diethyl toluene diamine) PA0 di(methylthio) toluene diamine PA0 1,3,5-triethyl- 2,6-diaminobenzene PA0 toluene diamine derivatives containing halogen groups, cyano groups, alkoxy, PA0 alkylthio, alkenyl or carbonylic moieties PA0 m-phenylene diamine PA0 p-phenylene diamine PA0 4'4 '-methylenedianiline PA0 4,4'-diaminodiphenyl sulfone PA0 2,6-diamino-pyridine PA0 4,4'-methylene bis (3-chloroaniline) PA0 4,4'-methylene bis(3-chloro-2,6-diethylaniline) PA0 4,4'-methylene bis (3-chloro-2,5-diethylaniline) PA0 3,3'-di-isopropyl-4,4'-diaminodiphenylmethane PA0 3,5,3', 5'-tetraethyl-4,4'-diaminodiphenylmethane PA0 propylene-di-4-aminobenzoate PA0 isobutyl 4-chloro-3,5-diaminobenzoate PA0 bis (2-aminophenyl) disulfide PA0 bis (4-aminophenyl) disulfide PA0 di(alkylated amino) benzene secondary diamine PA0 bis(alkylated aminophenyl) methane secondary diamine PA0 3,3'-carbomethoxy-4,4'-diamino diphenylmethane PA0 dimethylethylene diamine PA0 diethylethylene diamine PA0 dimethyl propylene diamine PA0 diethyl propylene diamine PA0 pentaethylene hexamine PA0 hydrazine PA0 ethylene diamine PA0 propylene diamine PA0 butylene diamine PA0 hexane diamine PA0 isophorone diamine PA0 dicyclohexylmethane diamine PA0 cyclohexane diamine PA0 pentaethylene hexamine PA0 piperazine PA0 2-methylpentamethylene diamine PA0 1,12-dodecane diamine PA0 bis-hexamethylene diamine and mixtures thereof